The present invention relates to electrofluidic devices that provide an optical response for the purpose of altering surface reflectivity, transmission through a surface, or creating 2D images for informational display.
Electrowetting has been a highly attractive modulation scheme for a variety of optical applications. For example, electrowetting has been used to provide optical switches for fiber optics, optical shutters or filters for cameras and guidance systems, optical pickup devices, optical waveguide materials, and video display pixels.
Conventional electrowetting displays include a colored oil that forms a film layer against an electrically insulating fluoropolymer surface. Underneath the fluoropolymer is a reflective electrode constructed from aluminum. This colored oil film layer provides coloration to the reflective surface below. When a voltage is applied between a water layer residing above the oil film layer and the electrode below the fluoropolymer, the oil film layer is broken up as the water electrowets the fluoropolymer. When the voltage is removed, the oil returns to the film layer geometry. While the oil film layer is broken up, the perceived coloration of the surface is that of the reflective electrode (white) whereas, when the oil is in the film state, the perceived coloration is that of the oil. Coloration of the oil is provided by including at least one dye. Conventional electrowetting technology can provide greater than 70% white state and a contrast ratio of up to 10:1.
However, these conventional electrowetting display technologies cannot provide a bistable state in the absence of voltage, have limited white state reflectance (i.e., has difficulty achieving the 80% reflectance of paper), are challenged when creating fully saturated colors, and present manufacturing difficulty because the operating voltage increases as the lateral dimensions of the fluid vessel decreases.
What is needed is a device that uses color fluids without large or multiple voltage source requirements while providing gray-scale switching between a high white state reflectance and fully saturated colors.